1. Field of the Invention
The present invention relates to a method and apparatus for venting residual vapors from a liquid storage vessel. More particularly, the present invention relates to a method and apparatus for venting residual vapors from a liquid storage vessel by introducing a gas to the storage vessel after removal of all liquids to provide a motive force to vent the vapors.
2. Description of the Prior Art
Volatile liquids, such as benzene, petroleum and the like, are often stored in tanks at bulk terminals, refineries and end-user facilities, and transported in tanks aboard barges or ships, tank trucks and rail cars. All such containers shall be referred to herein as liquid storage vessels. While resident in these liquid storage vessels, volatilization of the liquid occurs leaving residual vapors which must be removed before workmen can be permitted to enter the vessel and before the vessel can be filled with a different liquid.
Currently, such residual vapors are purged by flooding liquid storage vessels with a sufficient volume of water or air to entrain the vapors and carry them out of the vessel. The resulting mixture of diluted vapors, in many cases, are simply emitted to the atmosphere and surrounding water supply where they pollute the environment. Emissions handled in this manner lead to severe environmental hazards. For example, the inhalation of benzene vapors may cause depression of bone marrow activity, convulsions and paralysis. In addition, hydrocarbons are a major contributor to the formation of smog which has been proven to increase respiratory disorders among the population.
In addition to these environmental problems, water flushing facilities must overcome many economic hurdles. Adequate water for such facilities may be expensive due to limited water resources or to restrictions concerning the reuse or recycling of the water. If the water must be reused or recycled, it must be treated to remove contaminants that might pollute the environment or contaminate the next vessel to be flushed.
The environmental problems associated with air flushing could be eliminated by sending the mixture of vented air and vapors to a combustion device where the harmful vapors would be destroyed rather than emitted to the atmosphere. Unfortunately, as much as three times the liquid storage vessel volume of air must be cycled through the vessel to ensure that all of the residual vapors are purged from the vessel. Clearly, such a solution is impractical because of the large amount of air which would have to be heated in a combustion device before the volatiles they carry would be destroyed. The size of the collection piping and combustion equipment associated with such a process, in addition to the amount of fuel required to combust the vapors, similarly would be quite large, thereby prohibitively increasing the cost of such a process.
There have been several patents in the prior art which attempted to address the problem of removing vapors from storage tanks and collecting the gases which are forced out of the storage tank to reuse such gases for combustion.
U.S. Pat. No. 291,085 shows apparatus for removing flammable gases from oil tanks which includes devices for causing an induced current of air to pass into a storage tank above the surface of the fluid (such as fuel oil) and at the same time conduct displaced gases to a point where they may be used as fuel or discharged with safety into the atmosphere. The patent which issued in 1884 teaches the use of air as a medium for forcing gaseous vapors from a storage tank.
It has been learned over the past hundred years that air is an unsafe medium for use in cleansing storage tanks and also can result in corrosion of the tank. The device shown by the patent is relatively simple and primitive and does not include the safety features or efficient means for recapture of vapors for other use as is claimed by the present invention.
U.S. Pat. No. 1,918,100 shows a gas-gathering system which is basically a closed system in which vapors which collect in a storage tank are pumped into a secondary vapor storage tank partially filled with water and from the vapor storage tank are recaptured through a compression and condensing process to provide dry gas for other uses such as combustion. The patent states as its primary objective the provision of a method and apparatus for maintaining a hydrocarbon gas at all times within the storage tanks above the liquid levels thereof with the specific end in view of preventing air or oxygen from entering the tanks and mixing with the gases contained therein.
It should be noted at this point that this patent specifically teaches away from the method and apparatus of the 291,085 patent in that 291,085 teaches the use of air as a medium for moving vapors out of a storage tank, and 1,918,100 specifically provides a method to prevent air or oxygen from entering the tank and mixing with the gases.
Although the 1,918,100 patent is a more modern gas collection system apparatus and method, it does not show nor suggest the present invention which includes control of the flow of a purge medium to provide a laminar flow to create a continuous stratified interface between the volatile vapors and the purge medium. Nor does 1,918,100 teach or suggest any mechanism for detection of completion of the purging operation nor mixing with a high BTU material for later combustion. Nor does either prior art patent introduce gas at the bottom of the tank as is shown and claimed with respect to one embodiment of the present invention.